1. Field of the Invention
The present invention relates to a method for correcting the reduced mass flow rate of a compressor in an internal combustion engine turbocharged with a turbocharger.
2. Description of the Related Art
As is known, some internal combustion engines are provided with a turbocharger supercharging system, which may increase the power developed by the engine by exploiting the enthalpy of exhaust gases to compress the air aspirated by the engine and thus increase the volumetric intake performance.
A turbocharger supercharging system includes a turbocharger equipped with a turbine, which is arranged along an exhaust pipe to rotate at high speed under the propelling force of exhaust gases expelled from the engine, and with a compressor, which is driven in rotation by the turbine and is arranged along the air intake duct to compress the air aspirated by the engine. In a turbocharger supercharging system, it is advantageous to the keep the operating range of the turbocharger within a useful zone dependent on engine running conditions for both functional reasons (i.e. to avoid irregular or, in any case, low efficiency operation) and structural reasons (i.e. to avoid damaging the turbocharger). In order to limit the supercharging pressure (i.e. the pressure of the compressed air downstream of the compressor), a bypass duct regulated by a wastegate (valve) is arranged in parallel with the turbine; when the wastegate opens, part of the exhaust gases flow through the bypass duct and thus bypass the turbine, this resulting in a drop in the rotor's rotational speed and a consequent decrease in supercharging.
The wastegate is operated by a pneumatic actuator, which is in turn controlled by a solenoid control valve that enables regulating the action of the wastegate. The pneumatic actuator includes a sealed shell, which internally supports a flexible diaphragm that divides the sealed shell into two reciprocally fluid-tight chambers. The flexible diaphragm is mechanically connected to a rigid rod that operates the wastegate to control the opening and closing of the wastegate. A first chamber is connected to atmospheric pressure, while a second chamber is connected to the supercharging pressure and may also be connected to atmospheric pressure through a duct controlled by the proportional solenoid control valve, which is able to choke the duct between a closed position, in which the duct is completely closed, and a position of maximum opening.
A contrast spring is arranged in the first chamber such that it is compressed between a wall of the shell and the flexible diaphragm and rests against the flexible diaphragm on the side opposite to the rod. When the pressure difference between the two chambers is lower than an operating threshold (determined by the contrast spring preload), the rod keeps the wastegate in a completely closed position, while when the pressure difference between the two chambers is higher than the operating threshold, the contrast spring starts to compress under the thrust of the flexible diaphragm, which thus deforms and causes a displacement of the rod that consequently moves the wastegate towards the open position. By controlling the solenoid control valve, it is possible to connect the second chamber to atmospheric pressure through a variable-sized opening, and so it is possible to adjust the pressure difference between the two chambers that, in turn, causes the opening or closing of the wastegate. It is important to note that until the difference between the supercharging pressure and atmospheric pressure exceeds the operating threshold (equal to the preload generated by the contrast spring divided by the area of the flexible diaphragm), the wastegate may not be opened by the action exerted by the solenoid control valve (which may only reduce, and not increase, the difference between the supercharging pressure and atmospheric pressure).
In known internal combustion engines, a target supercharging pressure is generated that is used to cause operation of the wastegate by adding an open-loop contribution factor and a closed-loop contribution factor: the open-loop contribution factor is generated using an experimentally obtained control map, while the closed-loop contribution factor is provided by a PID regulator that attempts to cancel a pressure error, i.e. a difference between the target supercharging pressure and the actual supercharging pressure measured by a sensor.
However, the preload generated by the contrast spring of the pneumatic actuator has high structure dispersion, considerable thermal drift and also a certain time drift. Furthermore, the pneumatic actuator has considerable hysteresis, i.e. the behaviour of the pneumatic actuator varies significantly between the opening movement and the opposite closing movement. Consequently, the control map used to determine the closed-loop contribution factor is highly nonlinear and pursuing the target supercharging pressure proves to be complicated; thus, pursuing the target supercharging pressure in known internal combustion engines tends to have large overshoots or undershoots (i.e. the actual supercharging pressure significantly exceeds or drops below the target supercharging pressure) and thus causes oscillations, especially when the supercharging pressure is around the operating threshold, below which the wastegate may not be opened by the action exerted by the solenoid control valve.
Overshoots (i.e. peaks) in the supercharging pressure are particularly troublesome because they cause significant stress (and therefore potential damage over time) to the mechanical components of the internal combustion engine and because they generate both noise perceptible by the vehicle occupants and corresponding undesired oscillations in the drive torque generated by the internal combustion engine.
To reduce the size of the overshoots, it is possible to reduce the additional contribution factor of the PID regulator used to calculate the closed-loop contribution factor for operation of the wastegate.
For example, patent EP2314850 describes a method for controlling the wastegate including the steps of determining, in a design stage, a control law that provides a target opening for a control actuator of the wastegate as a function of a supercharging pressure; determining a target supercharging pressure; measuring an actual supercharging pressure; determining a first open-loop contribution factor for a target position of the control actuator of the wastegate with the control law and as a function of the target supercharging pressure; determining a second closed-loop contribution factor for the target position of the control actuator of the wastegate; calculating the target position for the control actuator of the wastegate by adding the two contribution factors; and controlling the control actuator of the wastegate so as to pursue the target position for the control actuator of the wastegate.
The step of determining the second closed-loop contribution factor contemplates determining a virtual position for control actuator of the wastegate with the control law and as a function of the actual supercharging pressure; calculating a position error by calculating the difference between the first open-loop contribution factor of the target position for the control actuator of the wastegate and the virtual position of the control actuator of the wastegate; and determining the second closed-loop contribution factor by processing the position error with a first regulator that attempts to cancel the position error.
However, the control method described in patent EP2314850 is quite robust, quick and devoid of oscillations only in operating conditions where there is no significant passage of air directly from the intake manifold to the exhaust of the internal combustion engine.
The object of the present invention is to provide a method for correcting the reduced mass flow rate of a compressor in an internal combustion engine turbocharged with a turbocharger, this correction method being devoid of the above-described drawbacks and, in particular, simple and inexpensive to implement.
As set forth in the present invention, a method for correcting the reduced mass flow rate of a compressor in an internal combustion engine turbocharged with a turbocharger is provided as claimed in the appended claims.